Duplex mill liners



June 1965 R. H. T. DIXON ETAL 3,

DUPLEX MILL LINERS Filed April 12, 1962 7 illlllm United States Patent O 3,186,650 DUTLEX NtllLL LEJERS Ronald H. T. Dixon, (Ihesteriicld, and Arthur .F. Rickard,

The present invention relates to liners for grinding apparatus, and more particularly to composite cast metal liners for tumbling mills.

It is Well known to make liners for tumbling mills, such as ball and rod mills, for carbidic cast iron containing nickel and chromium as disclosed, for example, by Taggart in his Handbook of Mineral Dressing, 1945. Such liners are ordinarily attached to the inside of a grinding mill shell by mechanical means, such as bolting. In operation, mill liners are often subject to high degrees of impact and abrasion, and substantial expenditures are necessitated for replacing fractured or worn out liners. Accordingly, liners are needed which can be produced at low cost and which are highly resistant to impact and abrasion. In the mineral grinding art, resistanceto impact is particularly needed to an increased degree in view of the trend toward autogenous grinding and the use of very large pieces of ore in tumble grinding mills, particularly those employed in carrying out aut-ogenous grinding processes. While carbidic cast iron liners have excellent hardness and resistance to abrasion, they suffer from the disadvantage that if there is not a perfect fit with the inside of the mill, breakage of the liner is apt to occur by impact of the mill burden on inadequately supported facial areas of the liners. It should be appreciated that the machining of abrasion-resistant carbidic cast irons is difficult and expensive, and that even though high machining costs are incurred in the manufacture of liners of such alloys, it is usually not commercially feasible to machine these liners with such precision that every liner perfectly fits the mill shell at all points. Other disadvantages of prior art carbidic iron liners are that local stresses set up in the liners where they are bolted to the mill shell may lead to fractures, and that when fractures develop in liners which are held in place by discontinuous means, pieces of the liners become loose and fall away from the mill shell. Although many attempts were made to overcome the foregoing difficulties and other disadvantages, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that the foregoing disadvantages of difficulty and expense in machining and fracture of the liners are largely overcome by a duplex composite liner having only the face formed of carbidic cast iron and having the back made of a softer, more machinable and tougher metal or alloy continuously metallurgically bonded to the face. The duplex liner of the invention is also advantageous in that it can be produced by a simple and inexpensive process.

It is an object of the present invention to provide improved liners for grinding apparatus.

It is a further object of the invention to provide an inexpensive mill liner having a continuous and substantially "ice homogeneous wear face of a highly abrasion-resistant metal continuously metallurgically bonded to a backing of a second metal characterized by good machinability and impact resistance.

It is also an object of the invention to provide an improved tumbling mill with liners having improved resistance to impact and abrasion.

The invention further contemplates providing a process for casting inexpensive duplex mill liners.

It is also the purpose of the invention to provide a duplex mill liner produced by a special casting process.

It is likewise within the contemplation of the invention to provide an improved process for grinding in a tumbling mill.

Other objects and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawing which depicts in cross section a duplex liner illustrative of the in vention bolted to a tumbling mill shell.

Broadly stated, the present invention contemplates duplex mill liners and tumbling mills having such duplex mill liners, wherein only the face of the liners is formed of carbidic cast iron and the back consists of a softer and tougher metal or alloy continuously metallurgically bonded to the face. This duplex liner of the invention has a uniformly abrasion-resistant face, the surface and interior of which are substantially homogeneous, i.e., uninterrupted by holes, cavities, bolts, inserts, etc. The relatively soft back can readily be machined to fit the inside of the mill and moreover serves to support the hard face uniformly under impact conditions. It is an advantage of the composite liners of the invention that if, under exceptional conditions, the carbidic layer cracks in service, it will tend to be held in place by the tough backing to which it is continuously metallurgically bonded. The continuous metallurgical bond referred to herein is a substantially oxide-free bond formed under non-oxidizing conditions by solidification of two metals cast in contact with each other along an interface therebetween, where the metal temperatures are controlled to provide that the contacting surfaces of both metals are concurrently molten. The combinations of metals in the duplex liners of the invention are combinations of metals having thermal coefficients of expansion which are not substantially different from each other. The liners of the invention are particularly inexpensive to produce because the face and back are simply bonded by casting without the necessity of special inserts in the mold or mechanical means for attaching the face and back together.

Referring to the drawing there is shown in cross section a duplex liner 1 of the invention having a face 2 metallurgically bonded at interface 3 to a back 4, the interface being a zone of mixed metal. The liner 1 is attached to the inside of tumbling mill shell 5 by bolt 6 in tapped hole 7.

The carbidic cast iron used for the face of the liner can be alloyed or unalloyed. Satisfactory cast irons for the face include cast irons having a composition containing about 2% to 4.5% carbon, about 0.1% to 3.5% silicon, about 0.1% to 2% manganese, up to about 8% nickel, up to about 28% chromium, and up to about 2% each of molybdenum and copper, with the balance substantially iron along with minor amounts of sulfur and phosphorus and other elements such as are normally present in commercially produced cast irons. The metallurgical structure of the carbidic cast iron is controlled by control of chemical composition, cooling rate, and heat treatment to provide that in the finished condition the structure comprises an alpha iron matrix wherein substantially all of the carbon not required to form the matrix is present as carbide.

In carrying the invention into practice, itis advantageous that the carbidiccast iron for the face be a nickel- ;chromium white cast iron containing, for example, about 1% to 15% chromium, about 2% to 4% carbon, about 0.1% to 2% silicon, about 0.1% to 1% manganese, about 2.5% to 8% nickel, up to about 2% copper, up to about 2% molybdenum, and not more than 0.2% each of sulfur and phosphorus, with the balance being essentially iron. Other abrasion-resistant metals suitable for the face are ordinary (unalloyed) white cast iron; 12% chromium, low-carbon, e.g., about 1.5% carbon, white cast iron; and 28% chromium, 25% carbon white cast iron.

The back of the liner can be of steel or flake graphite cast iron, but advantageously it consists of graphitic cast iron in which the graphite is wholly or predominantly in the spheroidal form. A particularly advantageous combination is a nickel-chromium White cast iron face and a spheroidal graphite cast iron back.

A suitable cast steel for the back is, for example, a ferritic or martensitic steel of a composition containing about 0.1% to 0.8% carbon, about 0.1% to 2% silicon, about 0.1% to 2% manganese and up to a total of about of part in the bottom of a suitably shaped mold and then casting the other part on top of it. Advantageously the face part is cast first, as it is then, possible to chill it by means of a chill of graphite or other suitable material of high thermal conductivity at the bottom of the mold. We prefer to use a mold having an ingate at the bottom and one or more further ingates in the sides just above the;

level of the interface between the two parts. The first metal is introduced by bottom pouring, and when it has partially solidified the second metal is poured in through the side gates.

To illustrate the preferred method of casting a composite liner according to the invention, the production of a liner with a nickel-chromium white cast iron face and a spheroidal graphite cast iron face will now be described by way of example;

Molten nickel-chromium cast iron having the composition 3.0% carbon, 0.4% silicon, 0.4% manganese, 0.08% sulfur, 0.02% phosphorus, 3.8% nickel, 2.0% chromium, balance iron, at a temperature of 1450 C. was poured via a bottom ingate into a mold the bottom of which was formed by a graphite chill block, until the surface of the molten metal reached the level indicated by the line 3 in the accompanying drawing. It was allowed to solidify until its surface had become pasty, and molten magnesium-treated cast iron having the composition 3.5% carbon, 2.4% silicon, 0.2% manganese, 0.02% sulfur, 0.03%

phosphorus and 0.06% magnesium, balance iron was then carefully poured in at a temperature of 1380 C. through ingates in the side of the mold opening immediately above the level of the interface to fill the mold, and the casting was then allowed toxsolidify; On removal from the mold and sectioning there was found to be a good crease in both toughness and machinability and dilution of the face may lead to reduction in the thickness of the hard, carbidic layer. Advantageously, the face comprises about three-quarters of the thickness of-the composite liner and the back the remaining quarter.

When other metals are used for the face and back of the mold it will be understood that appropriate pouring temperatures should be employed.

To ensure the formation of a goodmetallurgical bond between the face and back of the liner, the surface of the metal first poured must be clean and must not be allowed to oxidize before the second metal is poured on top of it. Oxidation may lead to'blowing at the interface between the metals. The use of a chill is therefore advantageous, as it reduces the time taken by the first metal to solidify sufficiently, but whether or not a chill is. employed it may be advantageous to cover the surface of thefirst metal with a flux which will prevent its oxidation. A suitable flux is borax. The flux maybe placed in the mold before pouring begins, or may be added in liquid form together with the first metal.

It will be understood that when the back of the liner is of cast iron of such composition that it can be graphitized by heat treatment the back may be cast white and the composite casting subsequently annealed to render the back graphitic. This however is only possible if the face of the liner is not also thereby graphitized or otherwise harmed. l

The composite casting may also be heat-treated after solidification in the conventional manner to refine the matrix of either of the metal layers. If necessary, the liner may be ground or machined to shape on the back face before it is used. The soft back 'layermay also be drilled and tapped to enable the liner to be bolted to the shell of the mill. I

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A tumbling mill liner comprising a uniformly abrasion-resistant, continuous and substantially homogeneous cast metal face of carbidic cast iron and a readily machinable impact-resistant cast metal back of metal selected from the group consisting of spheroidal graphite cast iron and cast steel continuously metallurgically bonded to said face by an interface zone characterized by a cast structure of mixed metal of said face and said back.

2. In a tumbling mill for grinding material under conditions subjecting the mill to impact and abrasion, said mill being of the type comprising, a mill shell and an abrasion-resistant liner attached to the inside ofthe mill shell, the improvement comprising a duplex, composite cast metal liner having a uniformly abrasion-resistant, continuous and substantially homogeneous liner face of carbidic cast iron and a readily machinable impact resistant back of a metal selected from the group consisting-of spheroidal graphite cast iron. and cast steel continuously metallurgically bonded to said face by an interface zone characterized by a caststru'cture of mixed metal of said face and said back, said back being fit and mechanically attached to the inside of the millshell to support the abrasion-resistant face uniformly under impact conditions and being continuously covered toward the inside of the mill by said face to protect said back from abrasion by the material being ground and said, duplex liner being characterized by improved resistance to impact and to detachment of the abrasion-resistant linen-face from the mill.

'3. A tumbling mill liner comprising a uniformly abrasion-resistant, continuous and. substantially homogeneous face of carbidic cast-iron and a readily machinable impact-resistant back of spheroidal graphite cast iron. and having between said face and said back a cast iron inter- 3,186,650 5 [face zone of mixed metal of said face and said back, said 2,566,103 zone forming a continuous metallurgical bond joining 2,835,008 said face to said :back. 2,909,385 2,949,247 References Cited by the Examiner 5 3,042 323 UNITED STATES PATENTS 5 Weston 241-183 X Otani 22- 201 Janelid 241 183 Rosenqvist et a1 241--1 83 Hall 241183 I. SPENCER OVERHOLSER, Primary Examiner.

ROBERT A. OLEARY, Examiner. 

1. A TUMBLING MILL LINER COMPRISING A UNIFORMLY ABRASION-RESISTANT, CONTINUOUS AND SUBSTANTIALLY HOMOGENEOUS CAST METAL FACE OF CARBIDIC CAST IRON AND A READILY MACHINABLE IMPACT-RESISTANT CAST METAL BACK OF METAL SELECTED FROM THE GROUP CONSISTING OF SPHEROIDAL GRAPHITE CAST IRON AND CAST STEEL CONTINUOUSLY METALLURGICALLY BONDED TO SAID FACE BY AN INTERFACE ZONE CHARACTERIZED BY A STRUCTURE OF MIXED METAL OF SAID FACE AND SAID BACK. 